1. Field of the Invention
The present invention relates to a method for manufacturing a solar cell, as well as to a solar cell manufactured according to this method.
2. Description of the Related Art
Silicon solar cells are often provided with a metallic coating over an entire surface for reflection and for charge collection on the back side. This backside metallic coating is made of, as a rule, aluminum-based, thick-film paste, which is printed between silver-based soldering surfaces, over a large surface. When sintered above 800° C., the aluminum partially alloys with the upper semiconductor surface by forming the low-melting point (577° C.) AlSi eutectic and recrystallizing, and in the process, it over-compensates for the existing n+-doping from the phosphorus diffusion that had previously occurred all-around, to form highly p-doped (p+-)doping (see F. Huster, 20th European Photovoltaic Solar Energy Conference, Jun. 6-10, 2005, Barcelona, Spain). When the base doping is p, the aluminum-doped, recrystallized surface layer forms a p+-BSF (back surface field) having a p+p-transition (high-low transition).
For several years, the same cell structure has also been produced on n-doped silicon, using a virtually identical process. Then, the above-mentioned, aluminum-doped surface of the back side becomes the p+-emitter, and the phosphorus-doped layer of the front side becomes the front surface field (FSF).
A disadvantage of the methods known from the related art is that the printed aluminum paste layer must be app. 40 μm thick (after the sintering), in order to obtain sufficiently deep alloy formation or aluminum doping depth. Due to the bimetallic effect between it and the silicon wafer, a reduction in the wafer thickness below the 180 μm typical up to now results in wafer deformation (so-called bow) that is no longer tolerable. High costs for the solar cell result from the necessary thickness of the silicon wafer and the amount of silicon consequently needed.
The screen-printed metallic coating of the back side has an imperfect reflection factor of only 65% for the long-wave portions of the sunlight, which penetrate to the back side. An effective reflectivity of >90% would increase the optical path length of the incident light and, therefore, the generation of electron-hole pairs (that is, the current) in the interior of the cell. Consequently, a marked gain in efficiency would be obtained.
In spite of the field passivation by heavy doping, a metallic surface, both that of an emitter and that of a back surface field (BSF), has a large charge-carrier recombination rate. In order to allow more effective passivation of the aluminum-doped surface of the back side, the thick, screen-printed aluminum layer needed as a dopant source and the AlSi eutectic layer formed between it and the semiconductor surface must be etched off. In the previously known methods, a large amount of hydrochloric acid is necessary for that purpose, due to the thickness of the layers to be etched off. This constitutes a large waste disposal problem.